Compound objective lens having two focal points and apparatus using the lens

Abstract: A focus control method to perform focus control on at least two kinds of information media having layers of different thicknesses using an optical lens having a plurality of regions corresponding to the different thicknesses and having a plurality of numerical apertures corresponding to the different thicknesses, the focus control method comprising: a step to change a distance between an information medium and the optical lens: and a step to start focus control when a focus error signal detected from the information medium surpasses a predetermined level so as to perform focus control corresponding to the different thicknesses.

Description: The present invention relates to a focus control method for use with at least two types of information media such as an optical medium or a magneto-optical medium like an optical disk or an optical card with the imaging optical system 2. DESCRIPTION OF THE RELATED ART: An optical memory technique has been put to practical use to manufacture an optical disk in which a pit pattern formed of a series of pits is drawn to record information. The optical disk is utilized as a high density and large capacity of information medium. For example, the optical disk is utilized for a digital audio disk, a video disk, a document file disk, and a data file disk. To record information on the optical disk and to reproduce the information from the optical disk, a light beam radiated from a light source is minutely converged in an imaging optical system, and the light beam minutely converged is radiated to the optical disk through the imaging optical system. Therefore, the light beam is required to be reliably controlled in the imaging optical system with high accuracy. The imaging optical system is utilized for an optical head apparatus in which a detecting system is additionally provided to detect the intensity of the light beam reflected from the optical disk. Fundamental functions of the optical head apparatus are classified into a converging performance for minutely converging a light beam to form a diffraction-limited micro-spot of the light beam radiated on the optical disk, a focus control in a focus servo system, a tracking control in a tracking serve system, and the detection of pit signals (or information signals) obtained by radiating the light beam on a pit pattern of the optical disk. The fundamental function of the optical head apparatus is determined by the combination of optical sub-systems and a photoelectric transfer detecting process according to a purpose and a use. Specifically, an optical head apparatus in which a holographic optical element (or hologram) is utilized to minimize and thin the optical head apparatus has been recently proposed. 2.1. PREVIOUSLY PROPOSED ART: Fig. 1 is a constitutional view of a conventional optical head apparatus proposed in Japanese Patent Application No. 46630 of 1991 which is applied by inventors of the present invention. As shown in Fig. 1, a conventional optical head apparatus 11 for recording or reproducing information on or from an information medium 12 such as an optical disk is provided with a light beam source 13 such as a semiconductor laser, a transmission type of blazed hologram 14 for transmitting a light beam L1 radiated from the light beam source 12 without any diffraction in an outgoing optical path and diffracting a light beam L2 reflected on the information medium 12 in a returning optical path, an objective lens 15 for converging the light beam L1 transmitting through the hologram 13 on the information medium 14 to read the information, an actuator 16 for integrally moving the objective lens 15 with the blazed hologram 13 to focus the light beam L1 on the information medium 12 with the objective lens 15, and a photo detector 17 for detecting the intensity of the light beam L2 reflected on the information medium 12 to reproduce the information. As shown in Fig. 2A, a relative position between the blazed hologram 14 and the objective lens 15 is fixed by a fixing means 18. Or, as shown in Fig. 2B, a blazed pattern is formed on a side of the objective lens 15 to integrally form the blazed hologram 14 with the objective lens 15. In the above configuration, a light beam L1 (or a laser beam) radiated from the light beam source 13 is radiated to the blazed hologram 14. and the light beam L1 mainly transmits through the blazed hologram 14 without any diffraction in an outgoing optical path. The light beam L1 transmitting through the blazed hologram 14 is called zero-order diffracted light. Thereafter, the zero-order diffracted light L1 is converged on the information medium 12 by the objective lens 15. In the information medium 12, information indicated by a series of patterned pits is recorded and read by the zero-order diffracted light L1. Thereafter, a beam light L2 having the information is reflected toward the objective lens 15 in a returning optical path and is incident to the blazed hologram 14. In the blazed hologram 14, the light L2 is mainly diffracted. The light L2 diffracted is called first-order diffracted light. Thereafter, the first-order diffracted light L2 is received in the photo detector 17. In the photo detector 17, the intensity distribution of the first-order diffracted light L2 is detected. Therefore, a servo signal for adjusting the position of the objective lens 15 by the action of the actuator 16 is obtained. Also, the intensity of the first-order diffracted light L2 is detected in the photo detector 17. Because the information medium 12 is rotated at high speed, the patterned pits radiated by the light 17 are changed so that the intensity of the first-order diffracted light L2 detected is changed. Therefore, an information signal indicating the information recorded in the information medium 12 is obtained by detecting the change in intensity of the first-order diffracted light L2. In the above operation, a part of the light beam L1 is necessarily diffracted in the blazed hologram 14 when the light beam L1 is radiated to the blazed hologram 14 in the outgoing optical path. Therefore, unnecessary diffracted light such as first-order diffracted light and minus first-order diffracted light necessarily occurs. In cases where the hologram 14 is not blazed, the unnecessary diffracted light in the outgoing optical path also reads the information recorded in the information medium 12, and the unnecessary light is undesirably received in the photo detector 17. To prevent the unnecessary light from transmitting to the information medium 12, the blazed hologram 14 is manufactured to form a blazed hologram pattern on the surface thereof, so that the intensity of the unnecessary light received in the photo detector 17 is decreased. Also, because an objective lens of a conventional microscope has only a focal point, images placed within a focal depth of the objective lens can be only observed with the conventional microscope. Also, a minute circuit is formed on a semiconductor such as a group III-V compound semiconductor to form a microwave circuit, an opto-electronic detector or a solid state laser. In this case, a photo-sensitive material is coated on a sample made of the semiconductor. Thereafter, a relative position between the sample and a photo mask covering the sample is adjusted by utilizing an alignment apparatus, and the photo-sensitive material is exposed by a beam of exposure light through the photo mask to transfer a circuit pattern drawn on the photo mask to the photo-sensitive material in an exposure process by utilizing an exposure apparatus. For example, an alignment pattern is drawn on a reverse side of the sample, and a relative position between the sample and the photo mask is adjusted with high accuracy while simultaneously observing the alignment pattern of the sample and the circuit pattern of the photo mask with the conventional microscope. Thereafter, the circuit pattern of the photo mask is transferred to a front side of the sample. In this case, because images placed within a focal depth of an objective lens utilized in the conventional microscope can be only observed with the conventional microscope, it is required to utilize the conventional microscope having a deep focal depth in the alignment apparatus in cases where the alignment pattern and the circuit pattern are simultaneously observed with the conventional microscope. Therefore, the magnification of the conventional microscope having a deep focal depth is lowered. 2.2. PROBLEMS TO BE SOLVED BY THE INVENTION: An optical disk having a high density memory capacity has been recently developed because of the improvement in a design technique of an optical system and the shortening of the wavelength of light radiated from a semiconductor laser. For example, a numerical aperture at an optical disk side of an imaging optical system in which a light beam converged on an optical disk is minutely narrowed in diameter is enlarged to obtain the optical disk having a high density memory capacity. In this case, the degree of aberration occurring in the imaging optical system is increased because an optical axis of the system tilts from a normal line of the optical disk. As the numerical aperture is increased, the degree of the aberration is enlarged. To prevent the increase of the numerical aperture, it is effective to thin the thickness of the optical disk. The thickness of the optical disk denotes a distance from a surface of the optical disk (or an information medium) radiated by a light beam to an information recording plane on which a series of patterned pits are formed. Fig. 3 shows a relationship between the thickness of the optical disk and the numerical aperture on condition that the tilt of the optical axis is constant. As shown in Fig. 3, because the numerical aperture is 0.5 when the thickness of the optical disk is 1.2 mm, it is effective to thin the optical disk to 0.6 mm in thickness when the numerical aperture is increased to 0.6. In this case, even though the numerical aperture is increased on condition that the tilt of the optical axis is not changed, the degree of the aberration is not increased. Therefore, it is preferred that the thickness of the optical disk be thinned to obtain the optical disk having a high density memory capacity. Accordingly, it is expected that the thickness of a prospective optical disk having a high density memory capacity becomes thinner than that of a present optical disk such as a compact disk appearing on the market now. For example, the thickness of the compact disk is about 1.2 mm, and the thickness of the prospective optical disk is expected to range from 0.4 mm to 0.8 mm. In this case, it is required to record or reproduce information on or from an optical disk with an optical head system regardless of whether the optical disk is the present optical disk or the prospective optical disk having a high density memory capacity. That is, an optical head apparatus having an imaging optical system in which a light beam is converged on an optical disk within the diffraction limit regardless of whether the optical disk is thick or thin is required. However, in a conventional optical head apparatus, a piece of information is only recorded or reproduced on or from an optical disk having a fixed thickness. For example, in cases where the thickness of the information medium 12 is off a regular range by about ±0.1 mm or more, an aberration such as a spherical aberration occurs when the optical head apparatus 11 is operated. Therefore, the recording or the reproduction of the information is impossible. Accordingly, there is a drawback that an optical head apparatus in which a piece of information is recorded or reproduced on or from an optical disk regardless of whether the optical disk is the present optical disk or the prospective optical disk having a high density memory capacity cannot be manufactured in a conventional technique. EP-0 470 807 discloses an optical disc apparatus in which different objective lenses are selected according to the thickness of the medium on or from which information is to be recorded or reproduced. According to the present invention, there is provided a focus control method to perform focus control on at least two kinds of information media having layers of different thicknesses using an optical lens having a plurality of regions each having a respective numerical aperture and corresponding to one of the different thicknesses, one of said regions comprising a hologram lens, the focus control method comprising: a step to change a distance between an information medium and the optical lens; and a step to start focus control when a focus error signal detected from the information medium reaches a predetermined level so as to perform focus control. The invention also provides an optical disk apparatus to perform focus control on at least two kinds of information media having layers of different thicknesses, comprising: an optical lens having a plurality of regions each having a respective numerical aperture corresponding to one of the different thicknesses, one of said regions comprising a hologram lens; and a focus controller to change a distance between an information medium and the optical lens and start focus control when a focus error signal detected from the information medium reaches a predetermined level so as to perform focus control. The features and advantages of the present invention will become apparent from the following description of exemplary embodiments and the accompanying drawings, in which:- Fig. 1 is a constitutional view of a conventional optical head apparatus proposed in Japanese Patent Application No. 46630 of 1991; Figs. 2A, 2B are respectively a cross sectional view of a set of an objective lens and a blazed hologram shown in Fig. 1; Fig. 3 shows a relationship between the thickness of an optical disk and a numerical aperture of an objective lens on condition that the tilt of the optical axis is constant; Fig. 4A is a constitutional view of an imaging optical system having a compound objective lens according to a first embodiment of the present invention, a beam of transmitted light not diffracted being converged on a thin type of information medium; Fig. 4B is a constitutional view of the imaging optical system shown in Fig. 4A, a beam of first-order diffracted light being converged on a thick type of information medium; Fig. 5 is a plan view of a hologram lens shown in Figs. 4A, 4B, a grating pattern of the hologram lens being depicted; Fig. 6 is a cross sectional view of the hologram lens shown in Fig. 5, the grating pattern formed in relief on the hologram lens being shown; Fig. 7 is an explanatory diagram showing an intensity distribution of transmitted light L4 converged on a converging spot S1 of a first information medium, a primary maximum and secondary maxima suppressed occurring in the converging spot S1; Fig. 8A is a cross sectional view of the hologram lens shown in Fig. 5, the grating pattern approximating to a step-wise shape composed of four stairs being shown; Fig. 8B is a cross sectional view of the hologram lens shown in Fig. 5, the grating pattern approximating to a step-wise shape composed of a plurality of stairs being shown; Fig. 9A is a constitutional view of an imaging optical system having a compound objective lens according to a modification of the first embodiment, a beam of first-order diffracted light being converged on a thin type of information medium; Fig. 9B is a constitutional view of the imaging optical system shown in Fig. 9A, a beam of transmitted light not diffracted being converged on a thick type of information medium; Fig. 10A is a constitutional view of an imaging optical system having a compound objective lens according to a second embodiment of the present invention, a beam of transmitted light not diffracted being converged on a thin type of information medium; Fig. 10B is a constitutional view of the imaging optical system shown in Fig. 10A, a beam of first-order diffracted light being converged on a thick type of information medium; Fig. 11 shows a change of a diffraction efficiency of a hologram lens shown in Figs. 10A, 10B; Figs. 12A to 12E are respectively a cross sectional view of the hologram lens shown in Figs. 10A, 10B, the grating pattern of the hologram lens approximating to a step-wise shape; Fig. 13A shows an intensity distribution of incident light utilized in the second embodiment, a far field pattern of the incident light being distributed in a Gaussian distribution: Fig. 13B shows an intensity distribution of transmitted light transmitting through a hologram lens shown in Figs. 10A, 10B, a far field pattern of the incident light being distributed in a gently-sloping shape; Figs. 14A to 14C show intensity distributions of transmitted light and diffracted light transmitting through a hologram lens shown in Figs. 10A, 10B; Fig. 15A is a plan view of a hologram lens according to a modification of the second embodiment, a grating pattern of the hologram lens being depicted; Figs. 15B, 15C are respectively a constitutional view of an imaging optical system having a compound objective lens according to another modification of the second embodiment; Fig. 16A is a constitutional view of an imaging optical system having a compound objective lens according to a third embodiment of the present invention, a beam of first-order diffracted light being converged on a thin type of information medium; Fig. 16B is a constitutional view of the imaging optical system shown in Fig. 16A, a beam of transmitted light not diffracted being converged on a thick type of information medium; Fig. 17 shows a change of a diffraction efficiency of a hologram lens shown in Figs. 16A, 16B; Figs. 18A to 18C show intensity distributions of transmitted light and diffracted light transmitting through a hologram lens shown in Figs. 16A, 16B; Fig. 19A is a cross sectional view of a compound objective lens according to a fourth embodiment of the present invention; Fig. 19B is a cross sectional view of a compound objective lens according to a modification of the fourth embodiment of the present invention; Fig. 20 is a cross sectional view of a compound objective lens according to a fifth embodiment of the present invention; Fig. 21 is a constitutional view of an optical head apparatus according to a sixth embodiment of the present Fig. 22 is a plan view of a wavefront changing device utilized in the sixth embodiment , a grating pattern of a hologram lens utilized as the wavefront changing device being depicted; Fig. 23 shows a positional relation between focal points of diffracted light occurring in the wavefront changing device shown in Fig. 22 and a photo detector; Fig. 24 is a plan view of a photo detector utilized in the sixth embodiment ; Fig. 25A and 25C respectively show a converging spot of first-order diffracted light radiated to detecting sections SE1, SE2 and SE3 of a sextant photo-detector shown in Fig. 24 and another converging spot of minus first-order diffracted light radiated to detecting sections SE4, SE5 and SE6 of the sextant photo-detector on condition that an objective lens shown in Fig. 21 is defocused on an information medium; Fig. 25B shows a converging spot of first-order diffracted light radiated to the detecting sections SE1, SE2 and SE3 of the sextant photo-detector and another converging spot of minus first-order diffracted light radiated to the detecting sections SE4, SE5 and SE6 of the sextant photo-detector on condition that the objective lens is just focused on the information medium; Fig. 26 shows a relationship between beams of diffracted light occurring in the wavefront changing device shown in Fig. 22 and the photo detector shown in Fig. 24; Fig. 27 is a constitutional view of an optical head apparatus according to a seventh embodiment; Fig. 28 is a plan view of a photo detector utilized in the seventh embodiment ; Figs. 29A, 29B, 29C show various shapes of converging spots converged on the photo detector shown in Fig. 28; Fig. 29D shows a radial direction Dr and a tangential direction Dt; Fig. 30 is a constitutional view of an optical head apparatus according to a first modification of the seventh embodiment; Fig. 31 is a constitutional view of an optical head apparatus according to a second modification of the seventh embodiment; Fig. 32 is a constitutional view of an optical head apparatus according to a third modification of the seventh embodiment; Fig. 33 is a constitutional view of an optical head apparatus according to afourthmodification of the seventh embodiment; Fig. 34 shows a beam of transmitted light not diffracted on an incoming optical path and a beam of transmitted light diffracted on the incoming optical path, the beams being utilized to detect an information signal; Fig. 35A graphically shows a change of a focus error signal obtained by detecting the intensity of transmitted light, the strength of the focus error signal depending on a distance between an objective lens and a first information medium; Fig. 35B graphically shows a change of a focus error signal obtained by detecting the intensity of diffracted light, the strength of the focus error signal depending on a distance between an objective lens and a second information medium. Fig. 36A graphically shows a change of a focus error signal obtained by detecting the intensity of diffracted light, the strength of the focus error signal depending on a distance between an objective lens and a first information medium; and Fig. 36B graphically shows a change of a focus error signal obtained by detecting the intensity of transmitted light, the strength of the focus error signal depending on a distance between an objective lens and a second information medium Preferred embodiments of a compound objective lens, an imaging optical system, an optical head apparatus, an optical disk, an optical disk apparatus, a binary focus microscope and an alignment apparatus according to the present invention are described with reference to drawings. (First Embodiment) Fig. 4A is a constitutional view of an imaging optical system having a compound objective lens according to a first embodiment of the present invention, a beam of transmitted light not diffracted being converged on a thin type of information medium. Fig. 4B is a constitutional view of the imaging optical system shown in Fig. 4A, a beam of first-order diffracted light being converged on a thick type of information medium. Fig. 5 is a plan view of a hologram lens shown in Figs. 4A, 4B, a grating pattern of the hologram lens being depicted. As shown in Figs. 4A, 4B, an imaging optical system 21 for converging light on a first substrate 22 of a thin type of first information medium 23 (a thickness T1) or a second substrate 24 of a thick type of second information medium 25 (a thickness T2) to form a diffraction-limited converging spot comprises a blazed hologram lens 26 for transmitting a part of incident light L3 radiated from a light source without any diffraction to form a beam of transmitted light L4 and diffracting a remaining part of the incident light L3 to form a beam of first-order diffracted light L5, and an objective lens 27 for converging the transmitted light L4 on the first information medium 23 or converging the first-order diffracted light L5 on the second information medium 25. The first information medium 23 represents a prospective optical disk having a high density memory capacity, and the thickness T1 of the first information medium 23 ranges from 0.4 mm to 0.8 mm. The second information medium 25 represents a compact disk or a laser disk appearing on the market now, and the thickness T2 of the second information medium 25 is about 1.2 mm. The term "convergence" denotes in this specification that divergent light or collimated light is focused to form a diffraction-limited micro spot. In the above configuration, a part of incident light L3 collimated transmits through the hologram lens 26 without any diffraction, and a beam of transmitted light L4 (that is, a beam of zero-order diffracted light L4) is formed. Thereafter, the transmitted light L4 is converged by the objective lens 27. Also, a remaining part of the incident light L3 is diffracted and refracted by the hologram lens 26, and a beam of first-order diffracted light L5 is formed. In this case, the hologram lens 26 selectively functions as a concave lens for the first-order-diffracted light L5, so that the first-order diffracted light L5 diverges from the hologram lens 26. Thereafter, the first-order diffracted light L5 is converged by the objective lens 27. In cases where the thin type of first information medium 23 is utilized to record or reproduce pieces of information on or from a front surface of the medium 23, as shown in Fig. 4A, the transmitted light L4 is incident on a rear surface of the first information medium 23 and is focused on its front surface by the objective lens 27 to form a diffraction-limited converging spot S1 on the first information medium 23. In contrast, in cases where the thick type of second information medium 25 is utilized to record or reproduce pieces of information on or from a front surface of the medium 25, the diffracted light L5 is incident on a rear surface of the second information medium 25 and is focused on its front surface to form a diffraction-limited converging spot S2 on the second information medium 25. Because the hologram lens 26 functions as a concave lens to diverge the first-order diffracted light L5, the diffraction-limited converging spots S1, S2 are formed even though the thickness T1 of the first information medium 23 differs from the thickness T2 of the second information medium 25. Therefore, a compound objective lens 29 composed of the hologram lens 26 and the objective lens 27 has substantially two focal points. As shown in Fig. 5, the hologram lens 26 is formed by drawing a grating pattern P1 in a pattern region 26A of a transparent substrate 28 in a concentric circle shape. The pattern region 26A is positioned in a center portion of the transparent substrate 28, and a no-pattern region 26B is positioned in a peripheral portion of the transparent substrate 28 to surround the pattern region 26A. An optical axis of the imaging optical system 21 passes through a central point of the grating pattern P1 and a central axis of the objective lens 27. In addition, as shown in Fig. 6, the grating pattern P1 of the hologram lens 26 is formed in relief to produce a phase modulation type of hologram lens. That is, blocks which each are composed of a bottom portion and a top portion are concentrically formed in the grating pattern P1. The height H of the relief in the grating pattern P1 is set to: H < λ/(n(λ)-1), where the symbol λ denotes a wavelength of the incident light L3 and the symbol n(λ) denotes a refractive index of the transparent substrate 28 for the incident light L3. In this case, a difference in phase modulation degree between the incident light L3 transmitting through a bottom portion of the grating pattern P1 and the incident light L3 transmitting through a top portion of the grating pattern P1 is lower than 2π radians. Therefore, a diffraction efficiency of the hologram lens 26 for the incident light L3 transmitting through the grating pattern P1 is less than 100 % to generate the light L4 transmitting through the grating pattern P1. Also, the incident light L3 transmitting through the no-pattern region 26B is not diffracted. As a result, the intensity of the transmitted light L4 can be sufficient to record or reproduce pieces of information on or from the first information medium 23. Also, because the intensity of the transmitted light L4 is sufficient over the entire surface of the hologram lens 26, secondary maxima (side lobes) of the transmitted light L4 undesirably occurring in the converging spot S1 can be suppressed. In detail, as an intensity distribution of the transmitted light L4 converged on the converging spot S1 is shown in Fig. 7, a primary maximum (a main lobe) of the transmitted light L4 positioned in a center of the converging spot S1 is utilized to record or reproduce a piece of information on or from the first information medium 23, and secondary maxima positioned around the primary maximum are unnecessary because the secondary maxima deteriorate a recording pit or a reproducing signal formed by the primary maximum. The grating pattern P1 of hologram lens 26 formed in relief is blazed as shown in Fig. 6, so that the occurrence of minus first-order diffracted light is considerably suppressed. Therefore, the intensity sum of the transmitted light L4 and the first-order diffracted light L5 is maximized. In other words, a utilization efficiency of the incident light L3 is enhanced. The numerical aperture NA of the objective lens 27 is equal to or more than 0.6. Also, when the transmitted light L4 is converged by the objective lens 27, the diffraction-limited converging spot S1 is formed on the first information medium 23 having a thickness T1. A diameter of the hologram lens 26 is almost the same as an aperture of the objective lens 27, so that a diameter of the pattern region 26A is smaller than the aperture of the objective lens 27. Because the incident light L3 transmitting through the no-pattern region 26B is not diffracted, not only the light L4 transmitting through the pattern region 26A but also the light L4 transmitting through the no-pattern region 26B are converged on the first information medium 23 by the objective lens 27 having a high numerical aperture. Therefore, the intensity of the transmitted light L4 converged at the converging point S1 can be increased. In contrast to the transmitted light L4, only the incident light L3 transmitting through the pattern region 26A of the hologram lens 26 is changed to the first-order diffracted light L5, and the first-order diffracted light L5 is converged on the second information medium 25 by the objective lens 27 having substantially a low numerical aperture. The phase of the light L4 transmitting through the grating pattern P1 of the pattern region 26A is determined by an average value of the phase modulation degrees in the light L4 transmitting through the bottom and top portions of the grating pattern P1. In contrast, because the height of the no-pattern region 26B is constant, the phase of the light L4 transmitting through the no-pattern region 26B is modulated at a phase modulation degree. Therefore, as shown in Fig. 6, the height of the no-pattern region 26B is set even with an average height of the grating pattern P1 to enhance the convergence function of the objective lens 27. For example, as shown in Fig. 8A, in cases where each block of the grating pattern P1 in the hologram lens 26 shown in Fig. 6 approximates to a step-wise shape composed of four stairs, a first step is etched at a depth h1+h2 and a width W1, a second step is etched at a depth h1 and a width W2, a third step is etched at a depth h2 and a width W2, and a fourth step is etched at a width W1. Therefore, the grating pattern P1 approximating to the step-wise shape is formed in the pattern region 26A. Thereafter, a peripheral portion of the transparent substrate 28 is etched by a depth h1 or h2 to form the no-pattern region 26B. Therefore, the height of the no-pattern region 26B is almost the same as an average height of the pattern region 26A, so that the phase of the light L4 transmitting through the pattern region 26A is almost the same as that of the light L4 transmitting through the no-pattern region 26B. In addition, as shown in Fig. 8B, an ideal blazed shape of the hologram lens 26 shown in Fig. 6 can approximate to a step-wise shape which is obtained by etching a center portion of the transparent substrate 28 many times. In this case, the height H0 of the step-wise shape is set to satisfy an equation H0 < λ/(n(λ)-1) so that the difference in phase modulation degree is set to a value lower than 2π radians. Specifically, in cases where the step-wise shape of the hologram lens 26 is composed of a flight of N stairs having the same difference n o in level, the difference n o in level is set to satisfy an equation n o < λ/{(n(λ)-1)*N} to set the difference in phase modulation degree of each stairs to a value lower than 2π/N radians. A peripheral portion of the transparent substrate 28 is etched to set the thickness of the no-pattern region 26B to a thickness of the pattern region 26A at one of the N.stairs which is not the top stair or the bottom stair. Therefore, the height of the no-pattern region 26B is almost the same as an average height of the pattern region 26A, so that the phase of the light L4 transmitting through the pattern region 26A is almost the same as that of the light L4 transmitting through the no-pattern region 26B. The grating pattern P1 of the hologram lens 26 is designed to correct any aberration occurring in the objective lens 27 and the second information medium 25, so that the first-order diffracted light L5 transmits through the second information medium 25 having a thickness T2 and is converged on the medium 25 to form the diffraction-limited converging spot S2 without any aberration. A method for designing the hologram lens 26 having an aberration correcting function is described. After the first-order diffracted light L5 is converged on the second information medium 25, spherical waves diverge from the converging spot S2 and transmit through the second substrate 24 and the objective lens 27. Thereafter, the spherical waves transmit through the transparent substrate 28 and optically interfere with the incident light L3. Therefore, an interference pattern is formed by the interference between the spherical waves and the incident light L3. The interference pattern can be calculated by subtracting the phase of the spherical waves from an inverted phase obtained by inverting the phase of the incident light L3. Accordingly, the grating pattern P1 of the hologram lens 26 which agrees with the interference pattern calculated can be easily formed according to a computer generated hologram technique. Accordingly, because the compound objective lens 29 is composed of the objective lens 27 and the hologram lens 26 in which a part of the incident light L3 is diffracted and refracted, a diffraction-limited converging spot can be reliably formed on an information medium regardless of whether the information medium has a thickness T1 or a thickness T2. Also, two diffraction-limited converging spots can be simultaneously formed on an information medium at difference depths. In other words, the compound objective lens has substantially two focal points. Also, because the diffraction efficiency of the hologram lens 26 is less than 100 % and the intensity of the light L4 transmitting through the hologram lens 26 is sufficient to record or reproduce information on or from the first information medium 23, the secondary maxima of the transmitted light L4 converged on the converging spot S1 can be suppressed. Also, because the hologram lens 26 is blazed, the occurrence of minus first-order diffracted light can be considerably suppressed. Therefore, the intensity sum of the transmitted light L4 and the first-order diffracted light L5 can be maximized, and a utilization efficiency of the incident light L3 can be enhanced. Also, because the hologram lens 26 functions as a lens only for the first-order diffracted light, the position of the converging point S1 formed by the transmitted light L4 differs from that of the converging point S2 formed by the first-order diffracted light L5 in an optical axis direction. Therefore, when the transmitted light L4 is converged in focus on an information recording plane of the information medium 23 to record or read a piece of information, the first-order diffracted light L5 converged on the information medium 23 is out of focus at the information recording plane. In the same manner, when the first-order diffracted light L5 is converged in focus on an information recording plane of the information medium 25, the transmitted light L4 converged on the information medium 25 is out of focus at the information recording plane.. Accordingly, when the light L4 (or L5) is converged on the converging spot S1 (or S2) in focus to record or read the information, the light L5 (or L4) not converged on the converging spot S1 (or S2) in focus does not adversely influence on the recording or reading of the information. To reliably prevent the adverse influence on the recording or reading of the information, a difference in the optical axis direction between the converging spots S1, S2 is required to be equal to or more than 50 µm. That is, when the difference is equal to or more than 50 µm, the light L5 (or L4) largely diverges to reduce the intensity of the light L5 (or L4) at an information recording plane when the light L4 (or L5) is converged on the converging spot S1 (or S2) of the information recording plane at a high intensity. Also, because the thickness T2 of the second information medium 25 representing the compact disk or the laser disk is about 1. 2 mm and because the thickness T1 of the first information medium 23 representing a prospective optical disk ranges from 0.4 mm to 0.8 mm, the difference in the optical axis direction between the converging points S1, S2 is required to be equal to or less than 1.0 mm by considering a moving range of an actuator with which the position of the compound objective lens 29 composed of the objective lens 27 and the hologram lens 26 is adjusted according to a focus servo signal. Because the hologram lens 26 functions as a concave lens for the first-order diffracted light, the difference between the converging points S1, S2 can be increased to about 1 mm. Accordingly, even though the transmitted light L4 and the first-order diffracted light L5 are simultaneously converged by the objective lens 27, no adverse influence is exerted on the recording or reproduction of the information on condition that the difference between the converging points S1, S2 ranges from 50 µm to 1 mm. Examples of the utilization of the imaging optical system 21 for various types of optical disks are described. In cases where the image optical system 21 is utilized for an optical disk device in which pieces of information recorded in a thin type of high density optical disk and a thick type of compact disk are exclusively reproduced, the diffraction efficiency of the hologram lens 26 for changing the incident light L3 to the diffracted light L5 is set in a range from about 20 % to 70 %. In this case, the intensity of the transmitted light L4 converged on the high density optical disk is almost the same as that of the first-order diffracted light L5 converged on the compact disk. Therefore, the output power of the incident light L3 can be minimized. Also, in cases where the image optical system 21 is utilized for an optical disk device in which pieces of information recorded in a thin type of high density optical disk are recorded or reproduced and pieces of information recorded in a thick type of optical disk are exclusively reproduced, the diffraction efficiency of the hologram lens 26 for changing the incident light L3 to the first-order diffracted light L5 is set to a value equal to or lower than 30 %. In this case, even though a high intensity of transmitted light L4 is required to record a piece of information on the high density optical disk, the recording of the information can be reliably performed without increasing the intensity of the incident light L3 because a transmission efficiency of the hologram lens 26 for the incident light L3 is high. In other words, a utilization efficiency of the incident light L3 can be enhanced when a piece of information is recorded on the high density optical disk, so that the output power of the incident light L3 can be minimized. In the first embodiment, the hologram lens 26 functions as a concave lens for the first-order diffracted light L5. However, it is applicable that a hologram lens 26M functioning as a convex lens for the first-order diffracted light L5 be utilized in place of the hologram lens 26. That is, as shown in Figs. 9A, 9B, the diffracted light L5 is converged on the first information medium 23 by the objective lens 27 to form the diffraction limited converging spot S1, and the transmitted light L4 is converged on the second information medium 25 by the objective lens 27 to form the diffraction limited converging spot S2. In this case, the difference between the converging points S1, S2 is required to be equal to or less than 0.5 mm by considering the moving range of the actuator. However, the occurrence of a chromatic aberration can be prevented in an imaging optical system 21M in which the hologram lens 26M functioning as a concave lens for the diffracted light L5 is utilized. The achromatization function in the imaging optical system is described in detail. When a focal length of the hologram lens 26M for the incident light L3 having a wavelength λ 0 is represented by f H0 and another focal length of the hologram lens 26M for the incident light L3 having a wavelength λ 1 is represented by f H1 , an equation (2) is satisfied. f H1 = f HO × λ O / λ 1 The focal length f H of the hologram lens 22 is shortened as the wavelength λ of the incident light L3 becomes longer. Also, when a refractive index of the objective lens 27 for the incident light L3 having a wavelength λ o is represented by n(λ o ) and another refractive index of the objective lens 27 for the incident light L3 having a wavelength λ 1 is represented by n(λ 1 ), a focal length f D (λ) of the objective lens 27 for the incident light L3 having a wavelength λ is formulated by an equation (3). f D (λ 1 ) = f D (λ O ) × (n(λ O ) - 1)/(n(λ 1 ) - 1) The focal length f D (λ) of the objective lens 27 is lengthened as the wavelength λ of the incident light L3 becomes longer. That is, the dependence of the focal length f D (λ) on the wavelength λ in the objective lens 27 is opposite to that of the focal length f H on the wavelength λ in the hologram lens 26M. Therefore, a condition that the compound objective lens 29M composed of the objective lens 27 and the hologram lens 26M functions as an achromatic lens is formulated by an equation (4). 1/f HO + 1/f D (λ O ) = 1/f H1 + 1/f D (λ 1 ) = 1/(f HO × λ O / λ 1 ) + (n(λ 1 ) - 1)/{f D (λ O ) × (n(λ O ) - 1)} Accordingly, because the dependence of the focal length f D (λ) on the wavelength λ in the objective lens 27 is opposite to that in the hologram lens 22, the compound objective lens 29M having an achromatic function can be formed by the combination of the lenses 26M, 27, and the occurrence of the chromatic aberration can be prevented. Also, even though the equation (4) is not strictly satisfied, the occurrence of the chromatic aberration can be largely suppressed. Also, a curvature of the objective lens 27 can be small because the hologram lens 26M functions as a convex lens for the first-order diffracted light L5. Also, because the hologram lens 26M is a plane type of element, a lightweight type of compound objective lens having an achromatic function can be made in large scale manufacture. A principal of the achromatization has been proposed in a first literature (D. Faklis and M. Morris, Photonics Spectra(1991), November p.205 & December p.131), a second literature (M.A. Gan et al., S.P.I.E.(1991), Vol.1507, p.116), and a third literature (P. Twardowski and P. Meirueis, S.P. I. E. (1991), Vol.1507, p.55) (Second Embodiment) Fig. 10A is a constitutional view of an imaging optical system having a compound objective lens according to a second embodiment of the present invention, a beam of transmitted light not diffracted being converged on a thin type of information medium. Fig. 10B is a constitutional view of the imaging optical system shown in Fig. 10A, a beam of first-order diffracted light being converged on a thick type of information medium. As shown in Figs. 10A, 10B, an imaging optical system 31 for converging light on the first substrate 22 of the first information medium 23 (the thickness T1) or the second substrate 24 of the second information medium 25 (the thickness T2) to form a diffraction-limited converging spot, comprises a blazed hologram lens 32 for transmitting a part of incident light L3 without any diffraction to form a beam of transmitted light L4 and diffracting a remaining part of incident light L3 to form a beam of first-order diffracted light L5, and the objective lens 27 for converging the transmitted light L4 on the first information medium 23 or converging the first-order diffracted light L5 on the second information medium 25. The hologram lens 32 is formed by drawing a grating pattern P2 in a pattern region 32A of the transparent substrate 28 in a concentric circle shape. The pattern region 32A is positioned in a center portion of the transparent substrate 28. An diameter of the grating pattern P2 is equal to or larger than an aperture of the objective lens 27. Also, a diffraction efficiency of the hologram lens 32 for the incident light L3 transmitting through the grating pattern P2 is less than 100 % in the same manner as in the first embodiment, so that the intensity of the transmitted light L4 is sufficient to record or reproduce a piece of information on or from the first information medium 23. In addition, as shown in Fig. 11, the diffraction efficiency in a central portion of the pattern region 32A is high, and the diffraction efficiency is gradually decreased toward an outer direction of the pattern region 32A. In other words, in cases where the grating pattern P2 of the hologram lens 32 is formed in relief, the height H of the relief in the grating pattern P2 is gradually lowered toward the outer direction of the pattern region 32A. Or, in cases where an ideal blazed shape of the hologram lens 26 approximates to a step-wise shape, each block of the grating pattern P2 positioned in the central portion of the transparent substrate 28 is formed in a step-wise shape shown in Fig. 12A in which an inclined angle  1 of stairs is large and a relationship W1>W2 between a first etching width W1 and a second etching width W2 is satisfied, and the grating pattern P2 formed in the step-wise shape shown in Fig. 12A is gradually changed by decreasing the first etching width W1 and increasing the second etching width W2 toward the outer direction of the pattern region 32A while the height H of the grating pattern P2 is gradually decreased. Therefore, each block of the grating pattern P2 positioned in a peripheral portion of the transparent substrate 28 is formed in a step-wise shape shown in Fig. 12B in which an inclined angle  2 of stairs is small and a relationship W1<W2 between the etching widths is satisfied. Also, each block of the grating pattern P2 positioned in a middle portion between the central and peripheral portions is formed in a step-wise shape shown in Fig. 12C in which the etching widths W1, W2 is the same. In the above configuration of the imaging optical system 31, a part of the incident light L3 transmits through the hologram lens 32 without any diffraction to form a beam of transmitted light L4, and the transmitted light L4 is converged by the objective lens 27. Also, a remaining part of the incident light L3 is diffracted and refracted by the hologram lens 32. In this case, the hologram lens 32 functions as a concave lens for the incident light L3, so that a first-order diffracted light L5 diverges from the hologram lens 32. Thereafter, the first-order diffracted light L5 is converged by the objective lens 27. In cases where the thin type of first information medium 23 is utilized to record or reproduce pieces of information on or from a front surface of the medium 23, as shown in Fig. 10A, the transmitted light L4 is incident on a rear surface of the first information medium 23 and is focused on its front surface by the objective lens 27 to form a diffraction-limited converging spot S3 on the first information medium 23. In this case, because the diffraction efficiency in the central portion of the grating pattern P2 is high and because the diffraction efficiency is gradually decreased toward the outer direction of the grating pattern P2, a diffraction probability of the incident light L3 is lowered in the peripheral portion of the grating pattern P2. Therefore, the light L4 transmits through the objective lens 27 on condition that the numerical aperture NA of the objective lens 27 is high. In contrast, in cases where the thick type of second information medium 25 is utilized to record or reproduce pieces of information on or from a front surface of the medium 25, the diffracted light L5 is incident on a rear surface of the second information medium 25 and is focused on its front surface to form a diffraction-limited converging spot S5 on the second information medium 25. In this case, because the hologram lens 32 functions as a concave lens to diverge the first-order diffracted light L5, the diffraction-limited converging spots S3, S4 are formed even though the thickness T1 of the first information medium 23 differs from the thickness T2 of the second information medium 25. Therefore, a compound objective lens 34 composed of the hologram lens 32 and the objective lens 27 has substantially two focal points. Accordingly, because the light L4 transmits through the objective lens 27 on condition that the numerical aperture NA of the objective lens 27 is high, the intensity of the transmitted light L4 converged on the first information medium 23 can be high. Also, in cases where the incident light L3 is radiated from a semiconductor laser, a far field pattern of the incident light L3 is distributed in a Gaussian distribution as shown in Fig. 13A. Therefore, because the diffraction efficiency is gradually decreased toward the outer direction of the grating pattern P2, a far field pattern of the transmitted light L4 is distributed in a gently-sloping shape as shown in Fig. 13B. In contrast to the second embodiment, because the incident light L3 is not diffracted in the no-pattern region 26b of the hologram lens 26 in the first embodiment, the intensity of the transmitted light L4 is suddenly increased at the peripheral portion of the hologram lens 26. Accordingly, secondary maxima of the transmitted light L4 converged on the converging spot S3 can be moreover suppressed in the second embodiment as compared with in the first embodiment. That is, the recording and reproducing of the information can be performed without any deterioration of the information by utilizing the imaging optical system 31. In addition, in cases where the first-order diffracted light L5 is converged on the second information medium 25 to form the diffraction-limited converging spot S4, a numerical aperture of the objective lens 27 for the first-order diffracted light L5 is low because the diffraction efficiency of the hologram lens 32 is decreased toward an outer direction of the pattern region 32A. As a result, the intensity of the first-order diffracted light L5 becomes lowered. In cases where the diffraction efficiency of the hologram lens 32 is heightened to increase the intensity of the first-order diffracted light L5, the intensity of the transmitted light L4 at its inner beam portion is largely decreased, and secondary maxima (or side lobes) of the transmitted light L4 at the converging spot S3 is undesirably increased. Therefore, the incident light L3 of which the far field pattern is distributed in the Gaussian distribution is radiated to the hologram lens 32 to increase the intensity of the first-order diffracted light L5 without any increase of the second maxima. In detail, as shown in Fig. 14A, the incident light L3 distributed not only in a central portion of the Gaussian distribution but also in a peripheral portion of the Gaussian distribution transmits through the hologram lens 32 and is refracted by the objective lens 27 because the diameter of the grating pattern P2 is equal to or larger than the aperture of the objective lens 27. Therefore, a numerical aperture NA of the objective lens 27 at a light source side for the incident light L3 becomes higher than that in the first embodiment, and the diffraction efficiency of the hologram lens 32 is heightened. As a result, the intensity of the first-order diffracted light L5 converged on the second information medium 25 can be increased, as shown in Fig. 14B. Also, because the intensity of the incident light L3 at the peripheral portion of the Gaussian distribution is low and because the diffraction efficiency of the hologram lens 32 is increased toward the inner direction of the grating pattern region 32A, intensity of the transmitted light L4 is distributed in a gently-sloping shape as shown in Fig. 14C. Accordingly, secondary maxima of the transmitted light L4 at the converging spot S3 can be suppressed. Examples of the utilization of the imaging optical system 31 for various types of optical disks are described. In cases where the image optical system 31 is utilized for an optical disk device in which pieces of information recorded in a thin type of high density optical disk and a thick type of compact disk are exclusively reproduced, the diffraction efficiency of the hologram lens 32 for the incident light L3 is set in a range from about 20 % to 70 %. In this case, the intensity of the transmitted light L4 converged on the high density optical disk is almost the same as that of the first-order diffracted light L5 converged on the compact disk. Therefore, the output power of the incident light L3 can be minimized. Also, in cases where the image optical system 31 is utilized for an optical disk device in which pieces of information recorded in a thin type of high density optical disk are recorded or reproduced and pieces of information recorded in a thick type of optical disk are exclusively reproduced, the diffraction efficiency of the hologram lens 32 for the incident light L3 is set to a value equal to or lower than 30 %. In this case, even though a high intensity of transmitted light L4 is required to record a piece of information on the high density optical disk, the recording of the information can be reliably performed without increasing the intensity of the incident light L3 because a transmission efficiency of the hologram lens 32 for the incident light L3 is high. In other words, a utilization efficiency of the incident light L3 can be enhanced when a piece of information is recorded on the high density optical disk, so that the output power of the incident light L3 can be minimized. In the second embodiment, the grating pattern P2 positioned in the central portion of the transparent substrate 28 is gradually changed toward the outer direction of the pattern region 32A from the step-wise shape shown in Fig. 12A to the step-wise shape shown in Fig. 12B through the step-wise shape shown in Fig. 12C. However, because the occurrence of unnecessary diffracted light such as minus first-order diffracted light can be effectively prevented in the middle portion of the transparent substrate 28 in which the grating pattern P2 is formed in the step-wise shape shown in Fig. 12C, it is preferred that the middle portion occupy a large part of the pattern region 32A of the hologram lens 32. In this case, the intensity sum of the transmitted light L4 and the first-order diffracted light L5 can be maximized, so that a utilization efficiency of the incident light L3 can be enhanced. Also, because the first etching width W1 of the grating pattern P2 is gradually decreased toward the outer direction of the pattern region 32A, it is applicable that the grating pattern P2 formed in the step-wise shape shown in Fig. 12B be changed to a step-wise shape shown in Fig. 12D when the first width W1 is decreased to a value lower than about 1 µm. That is, a flight of four stairs shown in Fig. 12B is changed to a flight of two stairs. In this case, the grating pattern P2 formed in the step-wise shape shown in Fig. 12D can be easily made. In addition, in cases where a height H4 of the grating pattern P2 formed in the step-wise shape shown in Fig. 12D is moreover decreased toward the outer direction of the pattern region 32A, it is preferred that the grating pattern P2 be formed in a step-wise shape shown in Fig. 12E. That is, a third etching width W3 is gradually decreased toward the outer direction of the pattern region 32A while decreasing a height H5 of the grating pattern P2. Therefore, the diffraction efficiency of the hologram lens 32 can be gradually decreased toward the outer direction of the pattern region 32A without any manufacturing difficulty of the grating pattern P2. In addition, as shown in Fig. 15A, it is applicable that a hologram lens 33 be formed, in place of the hologram lens 32, by placing the grating pattern P1 of the pattern region 32A in a central portion of the transparent substrate 28 and placing four types of diffraction regions 33A. 33B, 33C and 33D which surround the grating pattern P1. A part of the incident light L3 transmitting through each of the diffraction regions 33A to 33D is diffracted to control a transmission efficiency of the hologram lens 33. In this case, the intensity of the transmitted light L4 at its peripheral portion is decreased, so that secondary maxima occurring in the converging spot S3 can be suppressed. Also, it is applicable that the grating pattern P1 of the hologram be replaced with the grating pattern P2. Also, it is applicable that grating directions of the diffraction regions 33A to 33D differ from each other. In this case, even though the first-order diffracted light L5 diffracted in the diffraction region 33A is, for example, incident on the diffraction region 33c after the diffracted light L5 is reflected by the second information medium 25, the diffracted light L5 again diffracted in the diffraction region 33c does not pass in parallel to the optical axis. Therefore, in cases where a piece of information read from the second information medium 25 is detected in a detector to reproduce the information, the first-order diffracted light L5 diffracted in the diffraction regions 33A to 33D is not detected by the detector as stray light. Accordingly, the reproduction of the information does not deteriorate. Also, as shown in Fig. 15B, it is applicable that the hologram lens 32 function as a convex lens. In this case, the diffracted light L5 is converged on the first information medium 23, and the transmitted light L4 is converged on the second information medium 25, as shown in Fig. 15C. (Third Embodiment) Fig. 16A is a constitutional view of an imaging optical system having a compound objective lens according to a third embodiment of the present invention, a beam of first-order diffracted light being converged on a thin type of information medium. Fig. 16B is a constitutional view of the imaging optical system shown in Fig. 16A, a beam of transmitted light not diffracted being converged on a thick type of information medium. As shown in Figs. 16A, 16B, an imaging optical system 41 for converging light on the first substrate 22 of the first information medium 23 (the thickness T1) or the second substrate 24 of the second information medium 25 (the thickness T2) to form a diffraction-limited converging spot comprises a blazed hologram lens 42 for transmitting a part of incident light L3 without any diffraction to form a beam of transmitted light L4 and diffracting a remaining part of incident light L3 to form a beam of first-order diffracted light L6, and the objective lens 27 for converging the first-order diffracted light L6 on the first information medium 23 or converging the transmitted light L4 on the second information medium 25. The hologram lens 42 is formed by drawing a grating pattern P3 in a pattern region 42A of the transparent substrate 28 in a concentric circle shape. The pattern region 42A is positioned in a center portion of the transparent substrate 28. An diameter of the grating pattern P3 is equal to or larger than an aperture of the objective lens 27. Also, a diffraction efficiency of the hologram lens 42 for the incident light L3 transmitting through the grating pattern P3 is less than 100 % in the same manner as in the first embodiment, so that the intensity of the transmitted light L4 is sufficient to record or reproduce a piece of information on or from the second information medium 25. In addition, as shown in Fig. 17, the diffraction efficiency of the hologram lens 42 is high in a peripheral portion of the pattern region 42A, and the diffraction efficiency is gradually decreased toward an inner direction of the pattern region 42A. In other words, in cases where the grating pattern P3 of the hologram lens 42 is formed in relief, the height H of the relief in the grating pattern P3 is gradually lowered toward the inner direction of the pattern region 42A. Or, in cases where an ideal blazed shape of the hologram lens 26 approximates to a step-wise shape, each pitch of the grating pattern P3 positioned in the peripheral portion of the transparent substrate 28 is formed in a step-wise shape shown in Fig. 12A in which the inclined angle  1 of stairs is large and the relationship W1>W2 between the first and second etching widths W1, W2 is satisfied, and the grating pattern P3 formed in the step-wise shape shown in Fig. 12A is gradually changed by decreasing the first etching width W1 and increasing the second etching width W2 toward the inner direction of the pattern region 42A while the height H of the grating pattern P3 is gradually decreased. Therefore, each pitch of the grating pattern P3 positioned in a central portion of the transparent substrate 28 is formed in a step-wise shape shown in Fig. 12B in which the inclined angle  2 of stairs is small and the relationship W1<W2 is satisfied. Also, each pitch of the grating pattern P3 positioned in a middle portion between the central and peripheral portions is formed in a step-wise shape shown in Fig. 12C in which the etching widths W1, W2 is the same. In the above configuration of the imaging optical system 41, as shown in Fig. 16B, a part of the incident light L3 transmits through the hologram lens 42 without any diffraction to form a beam of transmitted light L4, and the transmitted light L4 is converged by the objective lens 27. Also, a remaining part of the incident light L3 is diffracted by the hologram lens 42 to form a beam of first-order diffracted light L6. In this case, the hologram lens 42 functions as a convex lens for the incident light L3, so that a first-order diffracted light L6 formed in the hologram lens 42 converges. Thereafter, the diffracted light L6 is converged by the objective lens 27. In cases where the thin type of first information medium 23 is utilized to record or reproduce pieces of information on or from a front surface of the medium 23, as shown in Fig. 16A, the diffracted light L6 is incident on a rear surface of the first information medium 23 and is focused on its front surface to form a diffraction-limited converging spot S5 on the first information medium 23. In contrast, in cases where the thick type of second information medium 25 is utilized to record or reproduce pieces of information on or from a front surface of the medium 25, the transmitted light L4 is incident on a rear surface of the second information medium 25 and is focused on its front surface to form a diffraction-limited converging spot S6 on the second information medium 25. In this case, because the hologram lens 42 functions as a convex lens to converge the diffracted light L6, the diffraction-limited converging spots S5, S6 are formed even though the thickness T1 of the first information medium 23 differs from the thickness T2 of the second information medium 25. Therefore, a compound objective lens 43 composed of the hologram lens 42 and the objective lens 27 has substantially two focal points. Also, because the hologram lens 42 functions as a convex lens for the diffracted light L6, the diffracted light L6 transmits through the objective lens 27 on condition that the numerical aperture NA of the objective lens 27 is substantially high. In addition, because the diffraction efficiency in the peripheral portion of the grating pattern P3 is high and because the diffraction efficiency is gradually decreased toward the inner direction of the grating pattern P3, a diffraction probability of the incident light L3 is higher in the peripheral portion of the grating pattern P3. The grating pattern P3 of the hologram lens 42 is designed to correct any aberration occurring in the objective lens 27 and the first information medium 23, so that the diffracted light L6 transmits through the first information medium 23 having the thickness T1 and is converged on the medium 23 to form the diffraction-limited converging spot S5 without any aberration. A method for designing the hologram lens 42 having an aberration correcting function is described. After the diffracted light L6 is converged on the first information medium 23, spherical waves diverge from the converging spot S5 and transmit through the first substrate 22 and the objective lens 27. Thereafter, the spherical waves transmit through the transparent substrate 28 and optically interfere with the incident light L3. Therefore, an interference pattern is formed by the interference between the spherical waves and the incident light L3. The interference pattern can be calculated by adding the phase of the spherical waves to an inverted phase obtained by inverting the phase of the incident light L3. Accordingly, the grating pattern P3 of the hologram lens 42 which agrees with the interference pattern calculated can be easily formed according to a computer generated hologram technique. Accordingly, because the hologram lens 42 functions as a convex lens for the first-order diffracted light L6, a curvature of the objective lens 27 can be lowered. Also, a glass material having a high refractive index is not required to produce the objective lens 27. Also, because the first-order diffracted light L6 formed in the hologram lens 42 converges before the diffracted light L6 is incident on the objective lens 27, the distance in an optical axis direction between the converging spots S5, S6 can be lengthened to about 1 mm. Therefore, even though the transmitted light L4 (or the first-order diffracted light L6) is converged on the converging spot S6 (or S5) in focus to record or read a piece of information, the light L6 (or L4) is not converged on the converging spot S6 (or S5) in focus to reduce the intensity of the light L6 (or L4) at the converging spot S6 (or S5). Accordingly, no adverse influence is exerted on the recording or reproduction of the information Also, because the hologram lens 42 functions as a convex lens for the first-order diffracted light L6, the occurrence of a chromatic aberration can be prevented in the imaging optical system 41. In detail, the focal length of the hologram lens 42 is shortened.as the wavelength of the incident light L3 becomes longer. In contrast, the focal length of the objective lens 27 is lengthened as the wavelength of the incident light L3 becomes longer. That is, the dependence of the focal length on the wavelength in the objective lens 27 is opposite to that of the focal length on the wavelength in the hologram lens 42. Therefore, the compound objective lens 43 having an achromatic function can be formed by the combination of the lenses 27, 42, and the occurrence of the chromatic aberration can be prevented. Also, because the hologram lens 42 is a plane type of element, a lightweight type of compound objective lens can be made in large scale manufacture. Also, because the diffraction efficiency of the hologram lens 42 is gradually decreased toward an inner direction of the pattern region 42A, the numerical aperture of the objective lens 27 for the first-order diffracted light L6 becomes substantially enlarged. Therefore, the intensity of the first-order diffracted light L6 can be enlarged to record or reproduce a piece of information on or from the first information medium 23. Also, in cases where the incident light L3 is radiated from a semiconductor laser, a far field pattern of the incident light L3 is distributed in a Gaussian distribution as shown in Fig. 13A. Therefore, because the diffraction efficiency of the hologram lens 42 is gradually decreased toward the inner direction of the grating pattern P2, a far field pattern of the first-order diffracted light L6 is distributed in a gently-sloping shape. Accordingly, secondary maxima of the first-order diffracted light L6 converged on the converging spot S5 can be moreover suppressed in the third embodiment as compared with in the first embodiment. That is, the recording and reproducing of the information can be performed without any deterioration of the information by utilizing the imaging optical system 41. In addition, in cases where the transmitted light L4 is converged on the second information medium 25 to form the diffraction-limited converging spot 56, a numerical aperture of the objective lens 27 for the transmitted light L4 is low because the diffraction efficiency of the hologram lens 42 is increased toward an outer direction of the grating pattern 42A. As a result, the intensity of the transmitted light L4 becomes lowered. In cases where a transmission efficiency of the hologram lens 42 is heightened to increase the intensity of the transmitted light L4, the intensity of the first-order diffracted light L6 at its inner beam portion is largely decreased, and secondary maxima (or side lobes) of the first-order diffracted light L6 at the converging spot S6 is undesirably increased. Therefore, the incident light L3 of which the far field pattern is distributed in the Gaussian distribution is radiated to the hologram lens 42 to increase the intensity of the transmitted light L4 without any increase of the second maxima. In detail, as shown in Fig. 18A, the incident light L3 distributed in not only a central portion of the Gaussian distribution but also a peripheral portion of the Gaussian distribution transmits through the hologram lens 42 and is refracted by the objective lens 27 because the diameter of the grating pattern P3 is equal to or larger than the aperture of the objective lens 27. Therefore, a numerical aperture NA of the objective lens 27 at a light source side for the incident light L3 becomes higher than that in the first embodiment, and a transmission efficiency of the hologram lens 42 is heightened. As a result, the intensity of the transmitted light L4 converged on the second information medium 25 can be increased, as shown in Fig. 18B. Also, because the intensity of the incident light L3 at the peripheral portion of the Gaussian distribution is low and because the diffraction efficiency of the hologram lens 42 is decreased toward the inner direction of the grating pattern 42A, the first-order diffracted light L6 is distributed in a gently-sloping shape as shown in Fig. 18C. Accordingly, secondary maxima of the first-order diffracted light L6 at the converging spot S5 can be suppressed. Examples of the utilization of the imaging optical system 41 for various types of optical disks are described. In cases where the image optical system 41 is utilized for an optical disk device in which pieces of information recorded in a thin type of high density optical disk and a thick type of compact disk are exclusively reproduced, the diffraction efficiency of the hologram lens 42 for the incident light L3 is set in a range from about 20 % to 70 %. In this case, the intensity of the transmitted light L4 converged on the compact disk is almost the same as that of the first-order diffracted light L6 converged on the high density optical disk. Therefore, the output power of the incident light L3 can be minimized. Also, in cases where the image optical system 41 is utilized for an optical disk device in which pieces of information recorded in a thin type of high density optical disk are recorded or reproduced and pieces of information recorded in a thick type of optical disk are exclusively reproduced, the diffraction efficiency of the hologram lens 42 for the incident light L3 is set to a value equal to or higher than 55 %. In this case, even though a high intensity of the first-order diffracted light L6 is required to record a piece of information on the high density optical disk, the recording of the information can be reliably performed without increasing the intensity of the incident light L3 because the diffraction efficiency of the hologram lens 42 for changing the incident light L3 to the first-order diffracted light L6 is high. In other words, a utilization efficiency of the incident light L3 can be enhanced when a piece of information is recorded on the high density optical disk, so that the output power of the incident light L3 can be minimized. Also, because the diffraction efficiency of the hologram lens 42 is gradually decreased toward an inner direction of the pattern region 42A, the numerical aperture of the objective lens 27 for the first-order diffracted light L6 becomes substantially enlarged. Therefore, the intensity of the first-order diffracted light L6 can be enlarged to record or reproduce a piece of information on or from the high density optical disk. In the third embodiment, the grating pattern P3 positioned in the pattern region 42A of the transparent substrate 28 is gradually changed toward the outer direction of the pattern region 42A from the step-wise shape shown in Fig. 12B to the step-wise shape shown in Fig. 12A through the step-wise shape shown in Fig. 12C while increasing the height H of the grating pattern P3. However, because the occurrence of unnecessary diffracted light such as minus first-order diffracted light can be effectively prevented in the middle portion of the transparent substrate 28 in which the grating pattern P3 is formed in the step-wise shape shown in Fig. 12C, it is preferred that the middle portion occupy a large part of the pattern region 42A of the hologram lens 42. In this case, the intensity sum of the transmitted light L4 and the first-order diffracted light L6 can be maximized, so that a utilization efficiency of the incident light L3 can be enhanced. Also, because the first etching width W1 of the grating pattern P3 is gradually decreased toward the inner direction of the pattern region 42A, it is applicable that the grating pattern P3 formed in the step-wise shape shown in Fig. 12B be changed to a step-wise shape shown in Fig. 12D when the first width W1 is decreased to a value lower than about 1 µm. In this case, the grating pattern P3 formed in the step-wise shape shown in Fig. 12D can be easily made. In addition, in cases where a height H4 of the grating pattern P3 formed in the step-wise shape shown in Fig. 12D is moreover decreased toward the inner direction of the pattern region 42A, it is preferred that the grating pattern P3 be formed in a step-wise shape shown in Fig. 12E. In this case, a third etching width W3 is gradually decreased toward the inner direction of the pattern region 42A while decreasing a height H5 of the grating pattern P3. Therefore, the diffraction efficiency of the hologram lens 42 can be gradually decreased toward the inner direction of the pattern region 42A without any manufacturing difficulty of the grating pattern P3. In the first to third embodiments of the image optical systems 21, 31 and 41, the grating patterns P1, P2 and P3 of the hologram lenses 26, 32 and 42 are respectively formed on a front side of the transparent substrate 28 not facing the objective lens 27. Therefore, a beam of light reflected at the front side of the transparent substrate 28 does not adversely influence as stray light on the recording or reproduction of the information. In detail, because the reflected light is diffracted by the hologram lens, the reflected light is scattered. Also, even though the first-order diffracted light L5 or L6 is reflected at a reverse side of the transparent substrate 28, the diffracted light reflected is again diffracted by the hologram lens and is scattered. Therefore, the light reflected at the front or reverse side of the hologram lens does not adversely influence on the recording or reproduction of the information. However, in cases where an anti-reflection film is coated on a front side of the hologram lens 28 at which the grating pattern is not formed, it is applicable that the grating patterns P1, P2 and P3 of the hologram lenses 26. 32 and 42 be respectively formed on a reverse side of the transparent substrate 28 facing the objective lens 27. In this case, because the first-order diffraction light L5, L6 is not refracted at the front side of the hologram lens 28, the design of the image optical systems 21, 31 and 41 can be simplified. Also, in the first to third embodiments, the grating patterns P1, P2 and P3 of the hologram lenses 26, 32 and 42 are respectively formed in relief to produce a phase modulation type of hologram lens. However, as is described in Provisional Publication No. 189504/86 (S61-189504) and Provisional Publication No. 241735/88 (S63-241735), the phase modulation type of hologram lens can be produced by utilizing a liquid crystal cell. Also, the phase modulation type of hologram lens can be produced by utilizing a birefringece material such as lithium niobate. For example, the phase modulation type of hologram lens can be produced by proton-exchanging a surface part of a lithium niobate substrate. (Fourth Embodiment) Also, in the first to third embodiments, the compound objective lens 29, 34 or 43 having two focal points is composed of the objective lens 27 and the hologram lens 26. 32 or 42. However, as a compound objective lens according to a fourth embodiment is shown in Fig. 19A, it is preferred that each of the hologram lenses 26, 32 and 42 and the objective lens 27 be unified with a packaging means 44 to form a compound objective lens 45 in which a relative position between each of the hologram lenses 26, 32 and 42 and the objective lens 27 is fixed. In this case, the transmitted light L4 and the first-order diffracted light L5, L6 can be easily converged on the first or second information medium 23, 25 by adjusting the position of the packing means 44 with an actuator. Also, as another compound objective lens according to a modified fourth embodiment is shown in Fig. 19B, it is preferred that each of the grating patterns P1, P2 and P3 be directly drawn on a curved side of the objective lens 27 facing a light source side to form a compound objective lens 46 in which each of the hologram lenses 26, 32 and 42 is integrally formed with the objective lens 27. Accordingly, the central axis of the objective lens 27 can always agree with that of each of the hologram lenses 26, 32 and 42, so that abaxial aberrations of each of the hologram lenses 26, 32 and 42 such as a coma aberration and an astigmatic aberration occurring in the first-order diffracted light can be prevented in the fourth embodiment. (Fifth Embodiment) Also, as a compound objective lens according to a fifth embodiment is shown in Fig. 20, it is preferred that each of the grating patterns P1, P2 and P3 be directly drawn on a side of the objective lens 27 facing the information medium 23 or 25 to form a compound objective lens 47 in which each of the hologram lenses 26, 32 and 42 is integrally formed with the objective lens 27. In this case, a curvature at the side of the objective lens 27 can be small or in a plane shape. Therefore, each of the grating patterns P1, P2 and P3 can be made at a low cost. Also, in cases where an aberration is caused by tilting the hologram lens from the optical axis, the aberration can be prevented by fixing the hologram lens and a light source of the incident light L3 on the same base. (Sixth Embodiment) An optical head apparatus with one of the compound objective lenses 29, 29M, 34, 43, 45, 46 and 47 shown in the first to fifth embodiments is described with reference to Figs. 21 to 26 according to a sixth embodiment of the present invention. X. Y and Z co-ordinates shown in Figs. 21 to 26 are utilized in common. Fig. 21 is a constitutional view of an optical head apparatus according to a sixth embodiment. As shown in Fig. 21, an optical head apparatus 51 for recording or reproducing pieces of information on or from the information medium 23 or 25, comprises a light source 52 such as a semiconductor laser for radiating the incident light L3, a collimator lens 53 for collimating the incident light L3, a beam splitter 54 for transmitting the incident light L3 on an outgoing optical path and reflecting a beam of transmitted light L4R formed by reflecting the transmitted light L4 on the information medium 23 or 25 or a beam of diffracted light L5R (or L6R) formed by reflecting the diffracted light L5 (or L6) on the information medium 23 or 25 on an incoming optical path, the compound objective lens 29 (or 29M, 34, 43, 45, 46 or 47) composed of the hologram lens 26 (or 26M, 32, 33 or 42) and the objective lens 27, a converging lens 55 for converging the transmitted light L4R or the diffracted light L5R reflected by the beam splitter 54, a wavefront changing device 56 such as a hologram for changing a wavefront of the transmitted light L4R or the diffracted light L5R to form a plurality of converging spots of the transmitted light L4R or the diffracted light L5R, a photo detector 57 for detecting intensities of the converging spots of the transmitted light L4R or the diffracted light L5R of which the wavefront is changed by the wavefront changing device 56 to obtain an information signal recorded on the information medium 23 or 25 and servo signals such as a focus error signal and a tracking error signal, and an actuating unit 58 for moving the compound objective lens composed of the hologram lens 26 and the objective lens 27 according to the servo signals. In the above configuration, a beam of incident light L3 radiated from the light source 52 is collimated in the collimator lens 53 and transmits through the beam splitter 54. Thereafter, a part of the incident light L3 transmits through the compound objective lens 29 without any diffraction, and a remaining part of the incident light L3 is diffracted. Thereafter, in cases where a piece of information is recorded or reproduced on or from the first information medium 23, the transmitted light L4 is converged on the first information medium 23 to form the first converging spot S1. That is, the transmitted light L4 is incident on a rear surface of the first information medium 23, and the first converging spot S1 is formed on a front surface of the first information medium 23. Thereafter, a beam of transmitted light L4R reflected at the front surface of the first information medium 23 passes through the same optical path in the reverse direction. That is, a part of the transmitted light L4R again transmits through the compound objective lens 29 without any diffraction and is reflected by the beam splitter 54. In this case, the transmitted light L4R is collimated. Thereafter, the transmitted light L4R is converged by the converging lens 55, and the wavefront of a large part of the transmitted light L4R is changed to form a plurality of converging spots on the photo detector 57. Thereafter, the intensities of the converging spots of the transmitted light L4R are detected in the photo detector 57. Therefore, an information signal and servo signals such as a focus error signal and a tracking error signal are obtained. The actuating unit 58 are operated according to the servo signals to move the compound objective lens 29 at high speed, so that the transmitted light L4 is converged on the first information medium 23 in focus. Also, in cases where a piece of information is recorded or reproduced on or from the second information medium 25, the diffracted light L5 is converged on the second information medium 25 to form the second converging spot S2. That is, the diffracted light L5 is incident on a rear surface of the second information medium 25, and the second converging spot' S2 is formed on a front surface of the second information medium 25. Thereafter, a beam of diffracted light L5R reflected at the front surface of the second information medium 25 passes through the same optical path in the reverse direction. That is, a part of the diffracted light L5R is again diffracted by the hologram lens 26 and is reflected by the beam splitter 54. In this case, the diffracted light L5R is collimated. Thereafter, the diffracted light L5R is converged by the converging lens 55, and the wavefront of a large part of the diffracted light L5R is changed to form a plurality of converging spots on the photo detector 57. In this case, the diffracted light L5R incident on the converging lens 55 is collimated in the same manner as the transmitted light L4R incident on the converging lens 55, the converging spots of the diffracted light L5R are formed at the same positions as those of the transmitted light L4R. Thereafter, the intensities of the converging spots of the diffracted light L5R are detected in the photo detector 57. Therefore, an information signal and servo signals such as a focus error signal and a tracking error signal are obtained. The actuating unit 58 are operated according to the servo signals to move the compound objective lens 29 at high speed, so that the diffracted light L5 is converged on the second information medium 25 in focus. In this case, because the transmitted light L4R again transmits through the compound objective lens without any diffraction and the diffracted light L5R is again diffracted by the hologram lens 26. the outgoing optical path agrees with the incoming optical path in a range between the information medium 23 or 25 and the beam splitter 54 even though the converging spot S1 differs from the converging spot S2. Therefore, a converging spot S7 on the photo detector 57 at which the light L4R or L5R not diffracted by the wavefront changing device 56 is converged relates to a radiation point of the light source 52 in a mirror image, so that the light L4R and L5R not diffracted by the wavefront changing device 56 are converged at the same converging point S7. In the same manner, the light L4R and L5R diffracted by the wavefront changing device 56 are converged at the same other converging points. Accordingly, even though the compound objective lens has two focal points, the wavefront changing unit 56 and the photo detector 57 required to detect the intensity of the transmitted light L4R can be utilized to detect the intensity of the diffracted light L5R. Therefore, the number of parts required to manufacture the optical head apparatus 51 can be reduced, and a small sized optical head apparatus can be manufactured at a low cost and in light weight even though pieces of information are recorded or reproduced on or from an information medium by utilizing the optical head apparatus 51 regardless of whether the information medium is thick or thin. In cases where the hologram lens 26 (or 32, 33, 42) is integrally formed with the objective lens 27 as shown in Fig. 19A, 19B or 20, each of the compound objective lenses 45. 46 and 47 can be manufactured in light weight because the hologram lens 26 (or 32, 33, 42) is a plane type of optical device. For example, the hologram lens 26 (or 32, 33, 42) is less than several tens mg in weight. Therefore, the hologram lens 26 integrally formed with the objective lens 27 can be easily moved by the actuating unit 58. Next, a detecting method of the servo signals is described. Fig. 22 is a plan view of the wavefront changing unit 56. Fig. 23 is an enlarged view of first-order diffracted light and transmitted light detected in the photo detector 57. As shown in Fig. 22, the wavefront changing unit 56 is partitioned into a diffracted light generating region 56a in which a grating pattern P4 is drawn and a pair of diffracted light generating regions 56b, 56c in which a pair of grating patterns P5, P6 are drawn. The light L4R or L5R incident on the diffracted light generating region 56a is diffracted to obtain a focus error signal. The light L4R or L5R incident on each of the diffracted light generating regions 56b, 56c is diffracted to obtain a tracking error signal. Initially, a spot size detection method utilized to detect a focus error signal is described as an example of a detecting method of a focus error signal. The method is proposed in Japanese Patent Application No. 185722 of 1990. In short, in cases where the method is adopted, an allowable assembly error in an optical head apparatus can be remarkably enlarged, and the servo signal such as a focus error signal can be stably obtained to adjust the position of the compound objective lens even though the wavelength of the incident light L3 varies. In detail, as shown in Fig. 23, the grating pattern P4 is designed to change the transmitted light L4R (or the diffracted light L5R) transmitting through the diffracted light generating region 56a of the wavefront changing unit 56 to a beam of first-order diffracted light L7 and a beam of minus first-order diffracted light L8. The diffracted light L7, L8 are expressed by two types of spherical waves having different curvatures. That is, interference fringes are produced by actually interfering a spherical wave having a focal point FP1 in the front of the photo detector 57 with another spherical wave diverging from the converging spot S7 according to a two-beam interferometric process, so that the grating pattern P4 agreeing with the interference fringes is formed. In other case, the interference fringes are calculated according to a computer generated hologram method. As a result, the transmitted light L4R (or the diffracted light L5R) transmitting through the diffracted light generating region 56a of the wavefront changing unit 56 is diffracted and changed to beams of conjugate diffracted light such as a beam of first-order diffracted light L7 and a beam of minus first-order diffracted light L8. The beam of first-order diffracted light L7 has the focal point FP1 at the front surface of the photo detector 57, and the beam of minus first-order diffracted light L8 has a focal point FP2 in the rear of the photo detector 57. As shown in Fig. 24, the photo detector 57 comprises a sextant photo-detector 59 (or a six-division photo detector) in which six detecting sections SE1, SE2, SE3. SE4, SE5 and SE6 are provided. The intensity of the first-order diffracted light L7 is detected by each of the detecting sections SE1, SE2 and SE3 of the sextant photo-detector 59 and is changed to electric current signals SC1, SC2 and SC3. Also, the intensity of the minus first-order diffracted light L8 is detected by each of the detecting sections SE4, SES and SE6 of the sextant photo-detector 59 and is changed to electric current signals SC4, SC5 and SC6. Fig. 25A and 25C respectively show a converging spot of the first-order diffracted light L7 radiated to the detecting sections SE1, SE2 and SE3 of the sextant photo-detector 59 and another converging spot of the minus first-order diffracted light L8 radiated to the detecting sections SE4, SE5 and SE6 of the sextant photo-detector 59 on condition that the objective lens 27 is defocused on the information medium 23 or 25. Fig. 25B shows a converging spot of the first-order diffracted light L7 radiated to the detecting sections SE1, SE2 and SE3 of the sextant photo-detector 59 and another converging spot of the minus first-order diffracted light L8 radiated to the detecting sections SE4, SE5 and SE6 of the sextant photo-detector 59 on condition that the objective lens 27 is just focused on the information medium 23 or 25. As shown in Figs. 25A to 25C, in cases where the transmitted light L4 (or the diffracted light L5) is converged on the information medium 23 (or 25) on condition that the objective lens 27 is defocused on the information medium 23 (or 25), a converging spot S8 of the diffracted light L7 shown at the left side of Figs. 25A, 25C is formed on the sextant photo-detector 59, and another converging spot S9 of the diffracted light L8 shown at the right side of Fig. 25A or 25C is formed on the sextant photo-detector 59. In contrast, in cases where the transmitted light L4 (or the diffracted light L5) is converged on the information medium 23 (or 25) on condition that the objective lens 27 is just focused on the information medium 23 (or 25), a converging spot S8 of the diffracted light L7 shown at the left side of Fig. 25B is formed on the sextant photo-detector 59, and another converging spot S9 of the diffracted light L8 shown at the right side of Fig. 25B is formed on the sextant photo-detector 59. The intensity of the diffracted light L7 is detected in each of the detecting sections SE1, SE2 and SE3 of the sextant photo-detector 59 and is changed to electric current signals SC1, SC2, SC3. Also, the intensity of the diffracted light L8 is detected in the detecting sections SE4, SE5 and SE6 of the sextant photo-detector 59 and is changed to electric current signals SC4, SC5 and SC6. Thereafter, a focus error signal S fe is obtained according to the spot size detection method by calculating an equation (5). S fe = (SC1 + SC3 - SC2) - (SC4 + SC6 - SC5) Thereafter, the position of the compound objective lens is moved in a direction along an optical axis at high speed so as to minimize the absolute value of the focus error signal S fe . In the spot size detection method, the diffracted light L7, L8 are expressed by two types of spherical waves having different curvatures to detect the focus error signal S fe . However, two beams of diffracted light L7, L8 radiated to the photo detector 57 are not limited to the spherical waves. That is, because the change of the diffracted light L7, L8 in a Y-direction is detected by the photo detector 57 according to the spot size detection method, it is required that a one-dimensional focal point of the diffracted light L7 is positioned in the front of the photo detector 57 and a one-dimensional focal point of the diffracted light L8 is positioned in the rear of the photo detector 57. Therefore, it is applicable that diffracted light including astigmatic aberration be radiated to the photo detector 57. In addition, an information signal S in is obtained by adding all of the electric current signals according to an equation (6). S in = SC1 + SC2 + SC3 + SC4 + SC5 + SC6 Because the information medium 23 or 25 is rotated at high speed, a patterned track pit radiated by the.converging spots S8, S9 of the diffracted light L7,L8 is rapidly changed one after another, so that the intensity of the information signal S in is changed. Therefore, the information stored in the information medium 23 or 25 can be reproduced according to the information signal S in . Next, the detection of a tracking error signal depending on a relative position between a converging spot and a patterned track pit on the information medium 23 or 25 is described. The grating pattern P5 drawn in the diffracted light generating region 56b shown in Fig. 22 is designed to change the transmitted light L4R (or the diffracted light L5R) transmitting through the diffracted light generating region 56b of the wavefront changing unit 56 to a beam of first-order diffracted light L9 and a beam of minus first-order diffracted light L10. Also, the grating pattern P6 drawn in the diffracted light generating region 56c shown in Fig. 22 is designed to change the transmitted light L4R (or the diffracted light L5R) transmitting through the diffracted light generating region 56c of the wavefront changing unit 56 to a beam of first-order diffracted light L11 and a beam of minus first-order diffracted light L12. As shown in Fig. 24, the photo detector 57 further comprises four tracking photo-detectors 60a to 60d for detecting intensities of the diffracted light L9 to L12. As shown in Fig. 26, the intensity of the diffracted light L9 is detected by the tracking photo-detector 60a and is changed to an electric current signal SC7, the intensity of the diffracted light L10 is detected by the tracking photo-detector 60d and is changed to an electric current signal SC10, the intensity of the diffracted light L11 is detected by the tracking photo-detector 60b and is changed to an electric current signal SC8, and the intensity of the diffracted light L12 is detected by the tracking photo-detector 60c and is changed to an electric current signal SC9. A tracking error signal S to is calculated according to an equation (7). S to = SC7 - SC8 - SC9 + SC10 Therefore, the asymmetry of the intensity distribution of the transmitted light L4R (or the diffracted light L5R) incident on the wavefront changing unit 56, which changes in dependence on the positional relation between the converging spot S1 (or S2) and a patterned track pit radiated by the light L4 or L5, is expressed by the tracking error signal S to . Thereafter, the objective lens 27 is moved in a radial direction so as to reduce a tracking error indicated by the tracking error signal S te . The radial direction is defined as a direction perpendicular to both the optical axis and a series of patterned track pits. Therefore, the converging spot S1 (or S2) of the transmitted light L4 (or the diffracted light L5) on the information medium 23 (or 25) can be formed in the middle of the patterned track pit, so that the tracking error becomes zero. Accordingly, focus and tracking servo characteristics can be stably obtained in the optical head apparatus 51. That is, because the wavefront changing unit 56 has a wavefront changing function, a focus error signal can be easily obtained. Also, because the diffracted light generating regions 56b, 56c are provided in the wavefront changing unit 56, a tracking error signal can be easily obtained. Therefore, the number of parts required to manufacture the optical head apparatus 51 can be reduced, and the number of manufacturing steps can be reduced. In addition, the optical head apparatus can be manufactured at a low cost and in light weight. Also, because the compound objective lens having two focal points is utilized in the optical head apparatus 51, pieces of information can be reliably recorded or reproduced from an information medium by utilizing the optical head apparatus 51 regardless of whether the information medium is thick or thin. (Seventh Embodiment) Next, an optical head apparatus in which servo signals such as a focus error signal and a tracking error signal are detected according to an astigmatic aberration method is described according to a seventh embodiment of the present invention. Fig. 27 is a constitutional view of an optical head apparatus according to a seventh embodiment. As shown in Fig. 27, an optical head apparatus 61 for recording or reproducing pieces of information on or from the information medium 23 or 25, comprises the light source 52. the collimator lens 53, the beam splitter 54, the compound objective lens 29 (or 29M, 34, 43, 45, 46 or 47) composed of the hologram lens 26 (or 26M, 32, 33 or 42) and the objective lens 27, the actuating unit 58, the converging lens 55, an astigmatic aberration generating unit 62 such as a plane parallel plate for generating an astigmatic aberration in the transmitted light L4R or the diffracted light L5R converged by the converging lens 55, and a photo detector 63 for detecting the intensity of the transmitted light L4R or the diffracted light L5R in which the astigmatic aberration is generated to obtain an information signal and servo signals such as a focus error signal and a tracking error signal. The astigmatic aberration generating unit 62 is classified into one of the wavefront changing unit 56 because a wavefront of the transmitted light L4R or the diffracted light L5R is changed by the generating unit 62 to generate the astigmatic aberration in the light L4R or L5R. Also, a normal line of the unit 62 is tilted from an optical axis. As shown in Fig. 28, the photo detector 63 comprises a quadrant photo-detector 64 in which four detecting sections SE7, SE8, SE9 and SE10 are provided. In the above configuration, the transmitted light L4R (or the diffracted light L5R) reflected by the information medium 23 (or 25) is converged by the converging lens 55 in the same manner as in the sixth embodiment. Thereafter, the transmitted light L4R (or the diffracted light L5R) transmits through the astigmatic aberration generating unit 62 and is converged on the photo detector 57 to form a converging spot S10 on the detecting sections SE7, SE8, SE9 and SE10 of the quadrant photo-detector 64. In this case, because the transmitted light L4R (or the diffracted light L5R) converged by the converging lens 55 is a spherical wave, an astigmatic aberration is generated in the transmitted light L4R (or the diffracted light L5R) by the astigmatic aberration generating unit 62. Therefore, as shown in Figs. 29A to 29C, the shape of the converging spot S10 considerably changes depending on a distance between the compound objective lens 29 and the information medium 23 (or 25). For example, in cases where the transmitted light L4 (or the diffracted light L5) is converged on the information medium 23 (or 25) on condition that the objective lens 27 is defocused on the information medium 23 (or 25), the converging spot S10 of the transmitted light L4R (or the diffracted light L5R) is formed on the quadrant photo-detector 64 as shown in Figs. 29A, 29C. In contrast, in cases where the transmitted light L4 (or the diffracted light L5) is converged on the information medium 23 (or 25) on condition that the objective lens 27 is just focused on the information medium 23 (or 25), the converging spot S10 of the transmitted light L4R (or the diffracted light L5R) is formed on the quadrant photo-detector 64 as shown in Fig. 25B. The intensity of the transmitted light L4R (or the diffracted light L5R) is detected in the detecting sections SE7, SE8, SE9 and SE10 of the quadrant photo-detector 64 and is changed to electric current signals SC11, SC12, SC13 and SC14. Thereafter, a focus error signal S fe is obtained according to an astigmatic aberration method by calculating an equation (8). S fe = (SC11 + SC14) - (SC12 + SC13) Thereafter, the position of the compound objective lens 29 is moved in a direction parallel to an optical axis at high speed so as to minimize the absolute value of the focus error signal S fe . Also, a tangential direction Dt agreeing with an extending direction of patterned recording pits and a radial direction Dr perpendicular to both the optical axis and the patterned recording pits are defined as shown in Fig. 29D. In this case, when the quadrant photo-detector 64 is directed as shown in Figs. 29A to 29C, a tracking error signal S te is calculated according to an equation (9) by utilizing an intensity distribution change of the transmitted light L4R (or the diffracted light L5R) which depends on a positional relation between the converging spot S10 and a recording pit radiated by the light L4 or L5. S te = SC11 + SC13 - (SC12 + SC14) Thereafter, the objective lens 27 is moved in the radial direction so as to reduce a tracking error indicated by the tracking error signal S te . Therefore, the converging spot S1, (or S2) of the transmitted light L4 (or the diffracted light L5) on the information medium 23 (or 25) can be formed in the middle of the recording pit, so that the tracking error becomes zero. In other case, the tracking error signal S te is obtained according to a phase difference method by utilizing the result calculated in the equation (8). In addition, an information signal S in is obtained by adding all of the electric current signals according to an equation (10). S in = SC11 + SC12 + SC13 + SC14 Accordingly, focus and tracking servo characteristics can be stably obtained in the optical head apparatus 61. That is, because an astigmatic aberration is generated in the transmitted light L4R (or the diffracted light L5R) by the astigmatic aberration generating unit 62 made of a plane parallel plate, the servo signals such as a focus error signal and a tracking error signal can be easily obtained. Therefore, the number of parts required to manufacture the optical head apparatus 61 can be reduced, and the number of manufacturing steps can be reduced. In addition, the optical head apparatus 61 can be manufactured at a low cost and in light weight. Also, because the compound objective lens having two focal points is utilized in the optical head apparatus 61, pieces of information can be reliably recorded or reproduced from an information medium by utilizing the optical head apparatus 61 regardless of whether the information medium is thick or thin. In the seventh embodiment, the astigmatic aberration generating unit 62 formed out of the plane parallel plate is arranged between the converging lens 55 and the photo detector 63. However, as an optical head apparatus 65 is shown in Fig. 30, it is applicable that a cylindrical lens 66 integrally formed with the converging lens 55 be arranged in place of the plane parallel plate to generate an astigmatic aberration in the transmitted light L4R (or the diffracted light L5R). In this case, because the cylindrical lens 66 is integrally formed with the converging lens 55, the optical head apparatus can be moreover manufactured at low cost. In addition, as shown in Fig. 30, it is applicable that a normal line of the hologram lens 26 (or 32. 33, 42) be tilted from an optical axis passing through the center of the objective lens 27 by about one degree to prevent stray light reflected in a surface of the hologram lens 26 from being incident on the photo detector 57 or 63. Also, it is applicable that the hologram lens 26 (or 32, 33, 42) be coated with an anti-reflection coating to prevent the occurrence of stray light. Also, as an optical head apparatus 67 is shown in Fig. 31, it is applicable that a polarized beam splitter 68 be arranged in place of the beam splitter 54 to perfectly transmit the incident light L3 and a 1/4-λ plate 69 be additionally placed between the hologram lens 26 (or 32, 33, 42) and the polarized beam splitter 68. In this case, because the incident light L3 transmits through the 1/4-λ plate 69 in an outgoing optical path and because the transmitted light L4R (or the diffracted light L5R) again transmits through the 1/4-λ plate 69 in an incoming optical path, the transmitted light L4R (or the diffracted light L5R) is perfectly reflected by the polarized beam splitter 68. Accordingly, a utilization efficiency of the incident light L3 can be enhanced. Also, a signal-noise ratio of each of the servo signals and the information signal can be enhanced. Also, as an optical head apparatus 70 is shown in Fig. 32, it is applicable that the polarized beam splitter 68 be arranged in place of the beam splitter 54 to perfectly transmit the incident light L3 and the 1/4-λ plate 69 be additionally placed between the hologram lens 26 (or 32, 33, 42) and the objective lens 27. In this case, the transmitted light L4R (or the diffracted light L5R) is perfectly reflected by the polarized beam splitter 68 in the same manner as the optical head apparatus shown in Fig. 31. In addition, because stray light reflected from the hologram lens 26 (or 32. 33. 42) transmits through the polarized beam splitter 68, the stray light is not incident on the photo detector 63. Accordingly, a signal-noise ratio of each of the servo signals and the information signal can be moreover enhanced. Also, as an optical head apparatus 71 is shown in Fig. 33, it is applicable that a wedge-like prism 72 for reshaping the incident light L3 radiated from the light source 52 be additionally placed between the collimator lens 53 and the polarized beam splitter 68. In this case, an elliptic wavefront of the incident light L3 is reshaped to a circular wavefront by the wedge-like prism 72. Accordingly, a utilization efficiency of the incident light L3 can be enhanced. In the sixth and seventh embodiments, when the transmitted light L4 (that is, zero-order diffracted light L4) converged on the first information medium 23 is reflected toward the compound objective lens to reproduce a piece of information recorded on the first information medium 23, a part of the transmitted light L4R is diffracted in the' hologram lens 26 (or 32, 33, 42) on the incoming optical path, so that the part of the transmitted light L4R is changed to a beam of first-order diffracted light L13. Therefore, the first-order diffracted light L13 diverges from the hologram lens 26, and a converging spot S11 of the diffracted light L13 is formed on the photo detector 57 or 63 in a relatively large size, as shown in Fig. 34. The size of the converging spot S11 is larger than those of the sextant photo-detector 59 and the quadrant photo-detector 64. Therefore, there is a drawback that a signal-noise ratio in the information signal deteriorates. To solve the drawback, it is preferred that the photo detector 57 (or 63) further comprise an information photo-detector 73 surrounding the sextant photo-detector 59 (or the quadrant photo-detector 64). The size of the information photo-detector 73 is equal to or larger than a 1mm square. Therefore, in cases where the information signal is determined by the sum of the intensity of the transmitted light L4 detected in the sextant photo-detector 59 (or the quadrant photo-detector 64) and the intensity of the diffracted light L13 detected in the information photo-detector 73, the signal-noise ratio in the information signal can be enhanced, and frequency characteristics of the information signal can be enhanced. (Eighth Embodiment) Next, a method of focusing performed in the optical head apparatuses 51, 61, 65, 67, 70 and 71 is described according to an eighth embodiment of the present invention. Fig. 35A graphically shows a change of the focus error signal obtained by detecting the intensity of the transmitted light L4 formed in the hologram lens 26, 32 or 33, the strength of the focus error signal depending on a distance between the objective lens 27 and the first information medium 23. Fig. 35B graphically shows a change of the focus error signal obtained by detecting the intensity of the diffracted light L5 formed in the hologram lens 26, 32 or 33, the strength of the focus error signal depending on a distance between the objective lens 27 and the second information medium 25. The intensity of the transmitted light L4 is high because the numerical aperture of the objective lens 27 for the transmitted light L4 is large. Therefore, as shown in Fig. 35A, a change of a focus error signal FE1 obtained in cases where the objective lens 27 is almost focused on the first information medium 23 is considerably large as compared with a change of an unnecessary focus error signal FE2 obtained in cases where the objective lens 27 is defocused on the.first information medium 23. In addition, in cases where the hologram lens 26, 32 or 33 is utilized in each of the optical head apparatuses 51, 61, 65, 67, 70 and 71, the unnecessary focus error signal FE2 is generated when the distance between the objective lens 27 and the first information medium 23 is larger than the focal length of the objective lens 27 for the transmitted light L4. In contrast, the intensity of the diffracted light L5 is comparatively low because the numerical aperture of the objective lens 27 for the diffracted light L5 is comparatively small. Therefore, as shown in Fig. 35B, a change of a focus error signal FE3 obtained in cases where the objective lens 27 is almost focused on the second information medium 25 is almost the same as that of an unnecessary focus error signal FE4 obtained in cases where the objective lens 27 is defocused on the second information medium 25. In addition, in cases where the hologram lens 26, 32 or 33 is utilized in each of the optical head apparatuses 51. 61, 65, 67, 70 and 71, the unnecessary focus error signal FE4 is generated when the distance between the objective lens 27 and the second information medium 25 is smaller than the focal length of the objective lens 27 for the diffracted light L5. Therefore, in cases where the focusing of the transmitted light L4 on the first information medium 23 is performed, the objective lens 27 placed far from the first information medium 23 is gradually brought near to the first information medium 23. Thereafter, when the strength of the focus error signal reaches a threshold value, a focus servo loop provided in the photo detector 57 or 63 is set to an operation condition, so that the objective lens 27 is set to be focused on the first information medium 23. Also, in cases where the focusing of the diffracted light L5 on the second information medium 25 is performed, the objective lens 27 placed far from the second information medium 25 is gradually brought, near to the second information medium 25 in the same manner. Thereafter, when the strength of the focus error signal reaches a threshold value, a focus servo loop provided in the photo detector 57 or 63 is set to an operation condition, so that the objective lens 27 is set to be focused on the second information medium 25. Accordingly, the inverse influence of the unnecessary focus error signal FE4 on the focusing of the diffracted light L5 can be prevented. Also, because the objective lens 27 placed far from the information medium 23 or 25 is gradually brought near to the information medium 23 or 25 regardless of whether the information medium is T1 or T2 in thickness, a focusing operation in each of the optical head apparatuses 51, 61, 65. 67, 70 and 71 with the hologram lens 26, 32 or 33 can be performed according to a common procedure by changing the threshold value or performing an auto gain control in which the focus error signal is normalized by detecting the total intensity of the transmitted light L4R or the diffracted light L5R. Therefore, a control circuit required to perform the focusing operation can be made at a low cost. Fig. 36A graphically shows a change of the focus error signal obtained by detecting the intensity of the diffracted light L6 formed in the hologram lens 42, the strength of the focus error signal depending on a distance between the objective lens 27 and the first information medium 23. Fig. 36B graphically shows a change of the focus error signal obtained by detecting the intensity of the transmitted light L4 formed in the hologram lens 42, the strength of the focus error signal depending on a distance between the objective lens 27 and the second information medium 25. As shown in Fig. 36A, a change of a focus error signal FE5 obtained in cases where the objective lens 27 is almost focused on the first information medium 23 is considerably large as compared with a change of an unnecessary focus error signal FE6 obtained in cases where the objective lens 27 is defocused on the first information medium 23. In addition, in cases where the hologram lens 42 is utilized in each of the optical head apparatuses 51, 61, 65, 67, 70 and 71, the unnecessary focus error signal FE6 is generated when the distance between the objective lens 27 and the first information medium 23 is smaller than the focal length of the objective lens 27 for the diffracted light L6. In contrast, as shown in Fig. 36B, a change of a focus error signal FE7 obtained in cases where the objective lens 27 is almost focused on the second information medium 25 is almost the same as that of an unnecessary focus error signal FE8 obtained in cases where the objective lens 27 is defocused on the second information medium 25. In addition, in cases where the hologram lens 42 is utilized in each of the optical head apparatuses 51, 61, 65, 67, 70 and 71, the unnecessary focus error signal FE8 is generated when the distance between the objective lens 27 and the second information medium 25 is larger than the focal length of the objective lens 27 for the transmitted light L4. Therefore, in cases where the focusing of the diffracted light L6 on the first information medium 23 is performed, the objective lens 27 placed near to the first information medium 23 is gradually moved away from the first information medium 23. Thereafter, when the strength of the focus error signal reaches a threshold value, a focus servo loop provided in the photo detector 57 or 63 is set to an operation condition, so that the objective lens 27 is set to be focused on the first information medium 23. Also, in cases where the focusing of the transmitted light L4 on the second information medium 25 is performed, the objective lens 27 placed near to the second information medium 25 is gradually moved away from the second information medium 25 in the same manner. Thereafter, when the strength of the focus error signal reaches a threshold value, a focus servo loop provided in the photo detector 57 or 63 is set to an operation condition, so that the objective lens 27 is set to be focused on the second information medium 25. Accordingly, the inverse influence of the unnecessary focus error signal FE8 on the focusing of the transmitted light L4 can be prevented. Also, because the objective lens 27 placed near to the information medium 23 or 25 is gradually moved away from the information medium 23 or 25 regardless of whether the information medium is T1 or T2 in thickness, a focusing operation in each of the optical head apparatuses 51, 61, 65, 67, 70 and 71 with the hologram lens 42 can be performed according to a common procedure by changing the threshold value or performing the auto gain control. Therefore, a control circuit required to perform the focusing operation can be made at a low cost. Having illustrated and described the principles of our invention in a preferred embodiment thereof, it should be readily apparent to those skilled in the art that the invention can be modified in arrangement and detail without departing from such principles. We claim all modifications coming within the scope of the accompanying claims.

Claims: A focus control method to perform focus control on at least two kinds of information media (23, 25) having layers of different thicknesses (T1, T2) using an optical lens (29) having a plurality of regions each having a respective numerical aperture corresponding to one of the different thicknesses, one of said regions comprising a hologram lens, the focus control method comprising: a step to change a distance between an information medium (23, 25) and the optical lens (29); and a step to start focus control when a focus error signal detected from the information medium reaches a predetermined level so as to perform focus control. A focus control method according to claim 1 comprising a step to standardize the focus error signal with an all sum signal detected from the information medium. An optical disk apparatus to perform focus control on at least two kinds of information media (23, 25) having layers of different thicknesses, comprising: an optical lens (29) having a plurality of regions each having a respective numerical aperture corresponding to one of the different thicknesses, one of said regions comprising a hologram lens; and a focus controller (57, 63) to change a distance between an information medium and the optical lens and start focus control when a focus error signal detected from the information medium reaches a predetermined level so as to perform focus control. An optical disk apparatus according to claim 3, wherein the regions of the lens are divided depending on differences in a distance from an optical axis of the lens, including a first region and a second region, the first region corresponding to a first thickness (T1), the second region located nearer to the optical axis than the first region, and the second region corresponding to the second thickness (T2) and said second thickness (T2) thicker than the first thickness (T1).

Inventors: KOMMA YOSHIAKI, MIZUNO SADAO, NISHINO SEIJI, KOMMA, YOSHIAKI, MIZUNO, SADAO, NISHINO, SEIJI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: G11B7
IPC codes: G02B21, G03F9, G11B19, G02B3, G11B7, G02B5